Synthetic imogolite

ABSTRACT

An inorganic material related to imogolite, a rare constituent of soil, which forms gels with under 1% solids concentration, is synthesized by digesting (at pH 3.1 to 5.0) soluble hydroxyaluminium complexes freshly formed at pH 3.2 to 5.5 in aqueous solution with restricted aluminium concentration.

This is a continuation of application Ser. No. 889,117, filed Mar. 22,1978, now U.S. Pat. No. 4,152,404.

This invention relates to an inorganic material, which is a fibrousproduct having tubular structure related to, or resembling, the naturalproduct imogolite.

Imogolite is a regular polymeric hydrated aluminium silicate which isfound principally in weathered volcanic deposits, often in the form ofthin gelatinous films, and consists of long tubes (for example 1 to 10micrometers) with an outside diameter of around 22 A and inside diameterabout 10 A. The tubes are partially aligned in bundles giving a highlyporous material (pore space around 60%) with pores of effectively about9 A diameter, affording a surface area of about 1000 m² g⁻¹, containingin the natural state from 10 to 45 g water per 100 g dry mineral atrelative humidity from 0.03 to 1. The water can be pumped off undervacuum or by heating in air to 150° C., the dried material remainingstable up to about 300° C.

The tubes can be dispersed by subjecting the material to ultrasonictreatment in an acidic solution of a pH of 3 to 5, and can bere-coagulated on adjusting the pH to 7.5 or more, this being the reverseof usual behaviour in clays.

On the basis of electron diffraction pattern, composition and the provenpresence of orthosilicate anions, it has been proposed that the walls ofthe tubes have a structure like that of a single sheet of gibbsite(Al(OH)₃) with orthosilicate groups replacing the inner hydroxyl surfaceof the gibbsite tube. This gives an empirical formula (HO)₃ Al₂ O₃ SiOH,which is also the sequence of atoms encountered on passing from theouter to the inner surface of the tubular model. Natural samples havecompositions in the range Al₂ O₃ (SiO₂)₁.0-1.2 (H₂ O)₂.3-3.0.

Therefore this invention consists of a method of synthesising aninorganic material which is a fibrous product having tubular structurerelated to, or resembling, the natural product imogolite, (for examplebeing 10-15% larger in tube diameter), the method comprising digesting asolution of pH from 3.1 to 5.0, containing soluble hydroxyaluminiumsilicate (preferably orthosilicate) complexes formed at pH 3.2 to 5.5 inaqueous solution, (the digestion being preferably in the presence ofsufficient dissolved silica to inhibit boehmite formation), theconcentration of Al (other than in already-formed imogolite or stablecomplexes) being not more than 50 (preferably not more than 25, morepreferably not more than 15) millimoles per liter, and the productdisplaying discernible electron diffraction peaks at 1.4 A, 2.1 A and4.2 A.

Further aqueous hydroxyaluminium silicate solution may be added in stepsduring the digestion to keep the concentration continuously high butwithin the above limits. Hydroxyaluminium silicate solutions are bestfreshly formed and kept cool. Preferably the adjusted-pH solution isheld at 40° C. to 170° C., preferably 90° C. to 130° C., conveniently95° C. to 100° C., at least until the yield of product reaches a maximum(typically taking 1 to 60 days.) Exemplary durations are 20 days at 60°C. and 1-3 days at 100° C. The temperature range is not mandatory, but,at excessive temperatures in the wet, imogolite decomposes to boehmiteor kaolinite and much silica remains in solution, while at lowertemperatures the reaction times become prolonged. The preferred pH offormation of the soluble hydroxyaluminium silicate complexes is around4.5, approached gradually from lower pH values. At lower pH, especiallyless than 4.0, formation of complexes is likely to be incomplete or slowunless the solution has experienced an excursion to higher pH values,whilst at higher pH, especially over 5.0, precipitates are likely toform, all the more so in more concentrated solutions. Any precipitateshould be redissolved or redispersed quickly to a pH more than 3.5.Redispersal may be in perchloric, hydrochloric, nitric, formic orbenzoic acids, preferably in a mixture of acetic with nitric orperchloric acids. Formation of synthetic imogolite at pH 4.7 to 5.0 iserratic and depends on the exact history of the solutions. Thus, in thatpH range, imogolite forms with difficulty if the solutions were tooalkaline and had been re-acidified or if the solutions were allowed toage. Therefore the preferred pH for the digestion is not more than 4.6,and this digestion pH may be reached by acidification withnon-complexing acids. Further, since the solution tends to become moreacidic during digestion, the preferred starting pH is not less than 3.5.

Heating excessively concentrated hydroxyaluminium silicate solutionsproduces weakly fibrous or non-fibrous products which do not form gels,and the situation cannot be retrieved by subsequent dilution. Thepreferred atomic ratio of Si to Al in the method is from 0.42 to 0.58,and should not exceed 0.8.

Chloride ion, if present, preferably does not exceed 25 millimolar.Acetic acid or similar weak organic acid is advantageously presentpreferably in a molar concentration at least half that of the aluminium,provided sufficient mineral acid, e.g. perchloric acid, is present toinhibit organic anion formation. The concentration of organic acidshould not exceed 25 mM, or twice that of the aluminium, whichever isthe greater. Concentrations more than the optimum can decrease the rateof formation of synthetic imogolite.

The product may be isolated from its colloidal solution, preferably bydrying, for example spray-drying, or by freeze-drying afterprecipitating a gel with alkali or added salt e.g. chloride or phosphateand centrifugir which may be repeated after washing and mechanicalagitation of the gel), or alternatively by foam-flotation using ananionic detergent. In this isolated form, it may find application as amolecular sieve, catalyst support, coagulant (i.e. gel-former) orsorbent.

Synthetic imogolite may incorporate other ions replacing Al or Si byisomorphous substitution (e.g. Cr(III) or Fe(III), or Ge or Tirespectively) and may be activated for catalysis by heating or exposingto hydrogen.

Coherent films can be formed by evaporating imogolite colloidalsolutions on to a flat surface; such films may find application asmembranes.

Instead of this isolation, the solution held for a while at 60° C. to140° C. may be made alkaline, for example with ammonia. A gel results,which may find application in its own right.

The product need not be isolated from its colloidal solution. Instead,for example, it may be used as a flocculant, a hydrophilizer or athickener.

The invention extends to the product of this method, optionally isolatedas set forth, to synthetic imogolite howsoever made, and to itsdispersions.

The invention also extends to a gel comprising synthetic imogolite,including a gel with a solids concentration of under 1% by weight e.g.under 0.5% e.g. 0.1%.

The invention will now be described by way of example.

EXAMPLE 1

An aqueous solution was prepared containing 1.4 millimolar silica SiO₂in the form of Si(OH)₄ monomer (silicic acid) and 2.4 millimolaraluminium trichloride AlCl₃. As chloride ions in excess of about 25millimolar appear to inhibit formation of imogolite, aluminiumtrichloride solutions should be not more concentrated than 8 mM. Onpartial neutralisation, the aluminium trichloride gives rise in solutionto hydroxyaluminium cations, which combine with the silicic acid to forma soluble hydroxyaluminium orthosilicate complex. A slight excess ofsilica over the theoretical requirement for imogolite was used toinhibit formation of boehmite (γ-AlOOH). The solution was adjusted to pH5 with 1 molar sodium hydroxide. Then 1000 ml of the solution wasacidified by adding 1 millimole of hydrochloric acid and 1 millimole ofacetic acid: the resultant pH was 4.35. The acidified solution washeated up to 95° C. in an inert vessel and maintained at thattemperature, either under reflux conditions or in a sealed pressurevessel, each giving the same results.

After 5 days the bulk of gel precipitated by ammonia reached a maximumand quantities of a synthetic mineral related to, or resembling, thenatural product imogolite had formed. The fibrous morphology, electrondiffraction pattern (with sharp peaks at 1.4 A, 2.1 A and 4.2 A) andinfrared spectrum all supported this conclusion.

The vessel containing the synthetic mineral had its contents renderedslightly alkaline by addition of ammonia; then the contents werecentrifuged. A bulky gel resulted, believed to incorporate an opennetwork of cross-linked synthetic-imogolite tubes, and having a solidscontent by weight of 0.1%.

EXAMPLES 2 to 9

Example 1 was repeated, except that the amount of the hydrochloric acidwas varied.

    ______________________________________                                               mmol HCl  Ini-   Time for                                                                             pH    Volume of                                Example                                                                              added per tial   maximum                                                                              There-                                                                              gel obtained                             No.    1000 ml   pH     gel bulk                                                                             after (arbitrary units)                        ______________________________________                                        2      0         4.55   5 days 4.0   14                                       3      0.5       4.45   5 days 3.6   24                                       4      0.68      4.1    3 days 3.5   20                                       5      1.02      4.0    3 days 3.4   20                                       6      1.35      3.9    3 days 3.3   20                                       7      2.04      3.8    3 days 3.2   19                                       8      2.72      3.7    3 days 3.2   18                                       9      3.40      3.6    3 days 3.2   15                                       ______________________________________                                    

EXAMPLES 10 to 15

Example 1 was repeated, except that the amount of acetic acid was variedsubstantially, under two different amounts of hydrochloric acid. Aceticacid can be seen to be inessential although advantageous. Also, thesolution containing Si(OH)₄ and AlCl₃ was adjusted to pH 5 before addingthe hydrochloric and acetic acids, and was then heated as in Example 1,to 95° C.

    ______________________________________                                                                        Days      Vol-                                     Millimoles                 for       ume                                 Ex.   of reagent per liter                                                                            Initial max. Final                                                                              of                                  No.  Si(OH).sub.4                                                                           AlCl.sub.3                                                                            HCl  HAc  pH    gel  pH   gel*                          ______________________________________                                        10   1.33     2.4     1    0    3.9   3    3.3  16                            11   1.33     2.4     1    2.2  4.1   5    3.5  18                            12   1.33     2.4     1    22.0 3.6   3    3.2  20                            13   1.33     2.4     2.5  0    3.6   5    3.2  18                            14   1.33     2.4     2.5  2.2  3.8   3    3.2  18                            15   1.33     2.4     2.5  22.0 3.5   3    3.2  20                            ______________________________________                                         *arbitrary units.                                                        

EXAMPLES 16 to 19

From these Examples, it will be seen that NaCl (preferably chloride iongenerally) inhibits imogolite formation and so limits the concentrationof AlCl₃ which can be used. Procedure was as in Example 10, except thatNaCl was added before the adjustment to pH 5. Note that 7 millimolesNaCl were unavoidably present derived from the AlCl₃ in each of Examples16-19 before any was added, so that the total amounts present were 7,17, 37 and 107 millimoles respectively.

    __________________________________________________________________________    Example                                                                            Millimoles reagent per liter                                                                     Initial                                                                           Days for                                                                           Final                                                                            Volume                                    No.  Si(OH).sub.4                                                                       AlCl.sub.3                                                                        NaCl                                                                              HCl                                                                              HAc                                                                              pH  max. gel                                                                           pH of gel                                    __________________________________________________________________________    16   1.33 2.4 0   1  2.2                                                                              4.5 2    3.3                                                                              15                                        17   1.33 2.4 10  1  2.2                                                                              4.5 5    3.3                                                                              14                                        18   1.33 2.4 30  1  2.2                                                                              4.5 5    3.2                                                                              7                                         19   1.33 2.4 100 1  2.2                                                                              4.4 5    3.2                                                                              4                                         __________________________________________________________________________

EXAMPLES 20 to 25

These Examples show that synthetic imogolite forms in the presence of0.1 M NaClO₄, and that Al(ClO₄)₃ of at least 9.6 mM can be used in thesynthesis. Procedure was as in Examples 16 to 19, but replacing AlCl₃ byAl(ClO₄)₃ and NaCl by NaClO₄. Solutions were diluted where necessary toequalise Al and Si concentrations before measuring yield of gel.

    __________________________________________________________________________    Millimoles reagent per liter                                                  Example         NaClO.sub.4 Initial                                                                           Days for                                                                           Final                                                                            Volume                                No.  Si(OH).sub.4                                                                       Al(ClO.sub.4).sub.3                                                                 added                                                                             total                                                                             HClO.sub.4                                                                        pH  max gel                                                                            pH of gel                                __________________________________________________________________________    20   1.33 2.4   0   7.2 1   4.4 1    3.2                                                                              12                                    21   2.66 4.8   0   14.4                                                                              2   4.3 5    3.2                                                                              20                                    22   4.00 7.2   0   21.6                                                                              3   4.3 5    3.3                                                                              20                                    23   5.33 9.6   0   28.8                                                                              4   4.4 5    3.7                                                                              14                                    24   2.66 4.8   30  44  2   4.3 7    3.4                                                                              17                                    25   2.66 4.8   100 114 2   4.3 7    3.8                                                                              15                                    __________________________________________________________________________

EXAMPLES 26 to 31

In these Examples, Example 1 was repeated except that the aluminium saltwas now aluminium iso-propoxide. Also, instead of hydrochloric andacetic acids, a variety of acids was used one at a time. The aluminiumiso-propoxide was added as a 0.1 M solution in iso-propanol to anaqueous Si(OH)₄ solution, then the acid was added and the solutionheated to 95° C.

    __________________________________________________________________________    Example                                                                            Millimoles per liter                                                                     Acid   Initial                                                                           Days for                                                                           Final                                                                            Volume                                     No.  Si(OH).sub.4                                                                       Al(iso-Pr).sub.3                                                                    (all 2 mmol)                                                                         pH  max gel                                                                            pH of gel                                     __________________________________________________________________________    26   2.66 4.8   none   5.5 --   5.4                                                                              0                                          27   2.66 4.8   HCl    4.4 2    3.4                                                                              14                                         28   2.66 4.8   HClO.sub.4                                                                           4.5 2    3.5                                                                              14                                         29   2.66 4.8   Acetic 4.3 2    4.1                                                                              14                                         30   2.66 4.8   Trichlor-                                                                            4.5 1    4.9                                                                              7                                                          acetic                                                        31   2.66 4.8   Oxalic 5.5 --   4.9                                                                              0                                          __________________________________________________________________________

Acids which failed (as did oxalic) include phthalic, citric and lactic,which all form strong aluminium complexes. Successful acids (such asHCl) also include formic and benzoic, which do not form significantaluminium complexes. Salicylic acid, which forms a weak aluminiumcomplex, was slightly successful. Presumably any complexant must bedisplaceable by silica if imogolite is to form.

EXAMPLES 32 to 34

These Examples show the use of alumina-silica precipitate to avoidexcess salt concentrations which inhibit imogolite formation. In each,500 ml of a sodium silicate solution, made from a fusion of 0.5 g quartzpowder in 2.5 g Na₂ CO₃, was slowly added to 100 ml of a solutioncontaining 15 mmol Al(ClO₄)₃ and 10 mmol HClO₄, giving a solution of pH4.4 containing a soluble hydroxyaluminium silicate complex. The solutionwas then adjusted to pH 5.5, and the resultant precipitate was separatedfrom the supernatant by centrifuging. The supernatant was discarded andthe precipitate immediately redispersed in 300 ml of 20 mM HClO₄. Thisdispersion and two dilutions from it were then heated at 95° C., whenthe precipitate rapidly redissolved. Synthetic imogolite formed in thediluted solutions as follows.

    ______________________________________                                        Solution composition,                                                         mmoles/liter                                                                                             Ini- Heating                                       Ex.                        tial time        Volume                            No.  SiO.sub.2                                                                            1/2(Al.sub.2 O.sub.3)                                                                  HClO.sub.4                                                                          pH   (days) Final                                                                              of gel*                           ______________________________________                                        32   1.7    4        2     4.6  5      3.2  19                                33   5.1    12       6     4.3  5      3.1  25                                34   17     40       20    4.1  5      3.5  11                                ______________________________________                                         *All volumes increased between 2 and 5 days. Gel volumes were measured on     solutions diluted to a standard 2.0 mmol Al/1000ml.                      

In further experiments (not described in detail) based on Examples32-34, alumina and silica were more completely precipitated fromsolutions adjusted to pH 6.5 or pH 8, rather than pH 5.5, and theprecipitates obtained were as satisfactory for imogolite synthesisprovided they were immediately redissolved in acid as described forExamples 32 to 34 above.

Other methods of reducing salt concentrations (seen from Examples 24 and24 to inhibit imogolite formation) should also be satisfactory inpromoting formation of imogolite: e.g. partial neutralisation of thealuminium-salt and silica solution with an anion-exchange resin in theOH form, rather than with NaOH, Na₂ CO₃, or the like; or ensuring thatthe salt formed is sparingly soluble (e.g. potassium perchlorate), sothat it can be removed by filtration, or using an aluminium alkoxide,e.g. with tetraalkyl silicate, instead of an inorganic aluminium salt.In further trials, Example 33 was repeated but, in place of theperchloric acid for redispersing the precipitate, there weresuccessfully used hydrochloric, nitric, formic and benzoic acids inturn. No gel was obtained when sulphuric acid was used, and a reducedyield was obtained with acetic acid alone. The best yields were obtainedwith a mixture of perchloric acid and acetic acid, and these conditionswere then standardised in later experiments.

EXAMPLES 35 to 39

Each of these Examples 35 to 39 has three versions, a 96° C., a 110° C.and a 120° C. version.

A preparation of a reactive alumina-silica sol is as follows: 5 litersof solution containing 1000 ppm SiO₂ (prepared as described below) wereadded over 30 minutes with vigorous stirring, to 150 m moles (56.27 g)Al(NO₃)₃.9H₂ O dissolved in 126 ml of 1 M HClO₄ and diluted to 1 liter.This gave 6 liters of a slightly opalescent sol which cleared in about20 minutes. After standing 1 hour, 1 M NaOH was added dropwise to pH4.5, when the sol was again allowed to clear by standing for 1 hour.Drop-wise addition of 1 M NaOH was then continued to pH 6.8. Theresulting precipitate was spun down in 6×1 liter polypropylene bottlesat 2000 rpm for 30 minutes. The clear supernatants were discarded, andthe precipitates combined by dispersing them in a solution containing 30ml 1 M HClO₄ and 43 ml of 1.74 M acetic acid, finally adjusting thetotal volume to one liter. This dispersion rapidly cleared.

The concentrated stock solution, containing 150 mM Al, about 80 mM Si,30 mM HClO₄ and 75 mM CH₃ COOH, was stored in a cold room and used insubsequent experiments. Its pH was around 3.9-4.2.

The 1000 ppm SiO₂ solution was prepared by fusing 5 g quartz with 25 ganhydrous Na₂ CO₃ and then dissolving the melt in 5 l distilled water.An alternative procedure for preparing the 1000 ppm SiO₂ solution wouldhave been to dissolve 22 g Na₂ SiO₃.9H₂ O (containing 5 g SiO₂ byanalysis) in 5 liters distilled water containing 160 ml 1 M Na₂ CO₃. Thepresence of the carbonate in this procedure helps in obtaining a clearreactive final dispersion.

In making the above stock solution, the silicate solution is added tothe acidified Al(NO₃)₃ solution, so that the reaction forming thealumina-silica complex proceeds always in acid solution. Reversing theorder of addition, i.e. adding the Al(NO₃)₃ solution (without addedacid) to the sodium silicate, gives a precipitate formed at pH over 7.This precipitate can also be dispersed in perchloric and acetic acid, asdescribed above to give opalescent concentrated stock solution, whichalso gives synthetic imogolite when diluted to 10 mM Al and heated at96° C., but at a lower yield.

The concentrated stock solution was diluted to give the concentrationsbelow, and then heated in inert plastics bombs in an oven at 96° C., orin an autoclave for 110° C. and 120° C. The results indicated that therewas little or no advantage in working at temperature above 100° C., andthat the yields of gel decreased at concentrations higher than 10 mM Al.An opalescence, probably due to boehmite, appeared at 120° C. in themore dilute solutions.

    __________________________________________________________________________    mmol Al   Volume of gel, measured on solutions diluted to 1 mmol Al/1000                ml.                                                                 Example                                                                            per 1000                                                                           120° C.                                                                            110° C.                                                                            96° C.                               No.  ml   day 1                                                                             day 2                                                                             day 3                                                                             day 1                                                                             day 2                                                                             day 3                                                                             day 1                                                                             day 2                                                                             day 3                               __________________________________________________________________________    35   10   13  16  16  12  17  20  13  18  20                                  36   15   13  16  16  10  15  17  10  14  16                                  37   20   10  11  12  6   11  12  6   10  12                                  38   25   7   10  10  5   9   10  5   7   9                                   39   30   7   8   8   4   6   7   4   5   6                                   __________________________________________________________________________

Gel volumes decreased rapidly at temperatures below 90° C. At 60° C., agel volume of only 6 units (measured at 2.5 m mol Al/1000 ml) wasobtained after incubating solutions containing 10 mM and 5 mM per 1000ml for as long as 1 month. This volume did not increase after longincubation.

A good yield of gel was however obtained at 60° C. by incubating asolution prepared as follows: A solution containing 2.5 mM Al(ClO₄)₃ and1.3 mM Si(OH)₄ (prepared by hydrolysis of tetraethoxysilane) wasadjusted to pH 5, then re-acidified (to pH 4.3) to give concentrationsof 0.5 mM perchloric acid and 1.25 mM acetic acid. After 21 days, thevolume of gel, measured on the undiluted solution, was 23 units.

EXAMPLE 40

Since gel formation is inhibited by high concentrations of reagent, aprocedure was developed in which the concentration of imogolite wasbuilt up in increments, each addition of the concentrated stock solutionof Example 35 being converted to synthetic imogolite under reflux beforemore reagent was added. In the present Example, the volumes of stocksolution added were adjusted to introduce 10 m mol/1000 ml of reactiveAl, and other components of the stock solution in proportion, at eachaddition. Initially (day 0), 50 ml stock solution was diluted to give750 ml containing 10 m mol/liter of Al, and the solution heated to boilgently under reflux. Subsequent additions of stock solutions were addedat 2-day intervals, and the gel volumes were monitored as below:

    ______________________________________                                        Before stock solution added                                                          mmol A1    Gel volume   Stock solution                                 Day    per liter  (1 mM Al)    increment ml                                   ______________________________________                                        2      10         20           54                                             4      19.4       23           57                                             6      28         22           62                                             8      34.6       28           66                                             10     42.3       25           71                                             12     49.5       21           76                                             14     56.3       20           81                                             16     62.5       18           87                                             18     68.3       18           93                                             20     73.8       17           --                                             28     73.8       20           --                                             ______________________________________                                    

Thus substantial concentrations of synthetic imogolite can be prepared.The pH of the starting solution was 4.5, but later remained in the range3.3-3.6 during the additions.

Other patterns of addition of stock solution attempting to increase theconcentration more rapidly gave lower gel volumes e.g. by increasing thefrequency of additions to one day intervals instead of two, or byincreasing the volumes of the stock solution added at each 2-dayincrement by 50%.

EXAMPLES 41 to 48

Examples 20 to 23 were repeated, but replacing Al(ClO₄)₃ and HClO₄ witheither Al(NO₃)₃ and HNO₃ (Examples 41 to 44), or AlCl₃ with HCl(Examples 45 to 48). These established that perchlorate is better thannitrate, and nitrate better than chloride.

    __________________________________________________________________________    mmol reagent per 1000 ml Days for  Volume of                                  Example                                                                            Si(OH).sub.4                                                                       Al(NO.sub.3).sub.3                                                                 HNO.sub.3                                                                         Initial pH                                                                          max. gel                                                                           Final pH                                                                           gel (2.5 mM)                               __________________________________________________________________________    41   1.33 2.4  1   4.6   1    3.2  14                                         42   2.66 4.8  2   4.5   5    3.3  20                                         43   4.00 7.2  3   4.4   7    3.8  14                                         44   5.33 9.6  4   4.4   7    3.8  8                                               Si(OH).sub.4                                                                       AlCl.sub.3                                                                         HCl                                                            45   1.33 2.4  1   4.7   2    3.2  15                                         46   2.66 4.8  2   4.6   2    3.2  15                                         47   4.00 7.2  3   4.6   5    3.6   8                                         48   5.33 9.6  4   4.5   5    3.7   7                                         __________________________________________________________________________

EXAMPLES 49 to 54

These Examples were performed to confirm that the atomic ratio of Si toAl in the reacting solutions should be near 0.5, preferably within thelimits 0.42-0.58 for optimum yields. Excess silica is preferable to toolittle silica. The general procedure followed that of Example 23.Examples 49 to 54 are according to the invention but 49, 50 and 54 arenon-preferred.

    __________________________________________________________________________                                         Volume                                   mmol reagent per 1000 ml  Days for                                                                           Final of gel                                   Example                                                                            Al(ClO.sub.4).sub.3                                                                 Si(OH).sub.4                                                                       HClO.sub.4                                                                        Initial pH                                                                          max. gel                                                                           pH Si:Al                                                                            (2.5 m Al)                               __________________________________________________________________________    49   10    7.5  5   4.3   2    3.8                                                                              0.75                                                                             7                                        50   10    6.7  5   4.3   2    3.7                                                                              0.67                                                                             11                                       51   10    5.8  5   4.3   2    3.5                                                                              0.58                                                                             18                                       52   10    5.0  5   4.3   2    3.3                                                                              0.50                                                                             20                                       53   10    4.2  5   4.3   2    3.1                                                                              0.42                                                                             17                                       54   10    3.3  5   4.3   1    3.1                                                                              0.33                                                                             6                                        __________________________________________________________________________

EXAMPLES 55 and 56

These Examples examine the nature of the products formed at highconcentrations of the stock solution described in Examples 35-39. Thisstock solution was heated undiluted and at various dilutions at 96° C.for 7 days:

    ______________________________________                                                                    Gel                                                                    pH     Volume                                            Example  mmoles reagent/liter                                                                            ini-   fi- (1 mM                                   No.      Al      Si     HClO.sub.4                                                                          HAc  tial nal Al)                               ______________________________________                                        55       10      5.3    2     5    4.3  3.5 23                                56       30      15.9   6     15   4.2  4.0 7                                 Comparative                                                                            90      47.7   18    45   4.1  4.1 0                                 Comparative                                                                            150     79.5   30    75   4.0  4.4 0                                 ______________________________________                                    

After heating, the undiluted stock solution was no longer reactive, andno gel formed when it was diluted to the 10 mM Al level and heated at96° C.

The products formed in the above four experiments were compared byelectron microscopy, electron diffraction, and infrared spectroscopy. Atthe 10 mM Al level well-ordered synthetic imogolite had formed with thecharacteristic diffraction pattern and morphology. At higherconcentrations of fibrous morphology was feebly developed or absent. Thecharacteristic diffraction features of synthetic imogolite at 1.4 A, 2.1A, and 4.2 A (associated with repeat distances along the fibre axis)became increasingly broad at 30 mM and not all were discernible at 90 mMAl or 150 mM Al. Certain broad diffraction bands of synthetic imogolite,most obviously those near 2.3 A and 3.45 A, persisted at allconcentrations, and were given also by the unheated stock solution.Stock solution diluted to the 10 mM Al level and heated to 60° C. for 1month gave a product with diffraction similar to the 90 mM Al level at96° C.

I claim:
 1. An inorganic material, which is a fibrous product having atubular structure resembling the natural product imogolite but having a10-15% larger tube diameter and displaying discernible electrondiffraction peaks at 1.4 A, 2.1 A and 4.2 A, the walls of said tubeshaving a structure like that of a single sheet of gibbsite withorthosilicate groups replacing the hydroxyl groups on the inner surfacesof said tubes.
 2. A membrane composed of the synthetic imogolitematerial of claim 1 cast into a coherent film.
 3. A colloidal solutionof the synthetic imogolite-like material of claim
 1. 4. The colloidalsolution of claim 3, prepared by:forming a soluble hydroxyaluminumsilicate complex by reacting soluble silica or a soluble silicatecompound with an aluminum compound capable of reacting with silica toform a soluble hydroxyaluminum silicate complex in an aqueous solutionat a pH of 3.2 to 5.5, the concentration of aluminum from said aluminumcompound other than already formed imogolite or stable hydroxyaluminumsilicate complex ranging up to 50 millimoles per liter and the Si:Alatomic ratio of the silica and aluminum compound reactants ranging from0.33 to 0.8; and digesting said aqueous solution of hydroxyaluminumsilicate at a pH of from 3.1 to 5.0 thereby forming a colloidaldispersion of said inorganic material.
 5. A gel of syntheticimogolite-like material of claim 1, wherein said gel possesses a solidsconcentration of less than 1% by weight.
 6. The gel of claim 5, whereinsaid solids concentration is less than 0.5%.
 7. The gel of claim 6,wherein said solids concentration is 0.1%.